Clutches and brakes are known which use frictionally locking connections as a functional principle. These brakes and/or clutches have, for example, at least one electrically excitable coil, and at least one element which can move in rotation about an axis, to bring about frictional locking with a corresponding mating element. In this context, the element which can move in rotation or else the corresponding mating element can move axially. The clutch and brake respectively transmit forces and torques, the brake reducing the kinetic energy of at least one moving body by means of a frictionally locking connection to at least one nonmoving body, and the clutch coupling to one another, and decoupling, at least two movable bodies for their common movement.
Brakes may, for example, be permanent magnet brakes. In this context, the force of a permanent magnetic field is used to generate the braking effect. Permanent magnets generate a permanent magnetic field and attract at least one rotationally movable element to a corresponding mating element for frictionally locking connection. To cancel the braking effect, an electromagnetic field with an at least partially opposed field direction is superimposed on the permanent magnetic field so that the fields can at least partially cancel one another out. The electromagnetic field can be generated by means of an electrically excitable coil. It is also possible to establish connections with this mechanism between the permanent magnetic field and the electromagnetic field.
Brakes may also be spring-pressure brakes. In such brakes, the force of an electromagnetic field is used to cancel out the force, generated by a spring on an element which can move in rotation about an axis. The spring presses, an element which is moved at least in rotation toward a corresponding mating element to form a frictionally locking connection. In order to cancel the braking effect, an electromagnetic field which brings about a magnetic force, which is opposed to the force of the spring, is generated by means of an electrically excitable coil. Connections may also be established with this mechanism between the spring force and magnetic force.
Permanent magnetic brakes, spring pressure brakes and spring pressure clutches have an electrically excitable coil in order to effect a quiescent current, a working current brake or clutch depending on the design of the brake or clutch. The electrically excited coil exerts a force on preferably ferromagnetic material, which is at least part of at least one of the elements used in the frictional locking, and which is located at a variable distance from the electrically excitable coil.
The frictional locking between the element which moves in rotation about an axis and the corresponding mating element results in abrasion, i.e. wear on at least one of the elements involved in the frictional locking. The wear is a limiting factor on the service life of the brake or the clutch. Depending on the degree of wear, the function of the brake or of the clutch may no longer be sufficiently reliable and accordingly, at least a part of the brake or the clutch will have to be replaced.
Hitherto, the user of the brake or of the clutch has determined the extent of the wear. Since abrasion is characteristic of wear, it is possible to measure the abrasion mechanically from time to time by means of a length meter. This procedure is both cumbersome, time-consuming and costly. To avoid the necessity of measuring the abrasion, it would be necessary to replace the brake or the clutch periodical. A further possibility is to count the number of braking operations, emergency stop operations and clutch activation operations. After a fixed number of operations, critical wear would be assumed and the brake or the clutch would be replaced. In this procedure, a large number of brakes and clutches would be unnecessarily replaced thus incurring costs, outage times, material and work time since the actual amount of wear would warrant replacement. Moreover, when wear occurs, depending on the type of brake or clutch, it is not absolutely necessary to replace the entire brake or the entire clutch, but rather only the one worn part. Also it may be possible to readjust the brake or the clutch in order to compensate for the wear.